1. Field of Invention
This invention relates to an apparatus for determining a frequency-dependent signal-to-noise ratio in a communications network so as to allow proper equalization in a transmit pre-emphasis mode.
2. Description of the Prior Art
It is well-known in the prior art that a transmitter in a communications network, particularly a multipoint network, should emphasize or amplify certain frequencies so as to compensate for frequency-dependent losses in the communications process. For example, with the use of a telephone line as a communications line, losses are more pronounced at higher frequencies. These losses are typically modelled as a constant negative slope above a given break frequency on a decibels versus frequency plot.
When noise is injected into a communications line subsequent to the high-frequency roll-off of the communications line and the signal rolls off above a break frequency while the noise signal remains constant (thereby resulting in a signal-to-noise ratio which progressively decreases above the break frequency), prior art methods of frequency-dependent analysis of total energy received is adequate as an equalization technique. These methods include fixed pre-emphasis wherein a fixed frequency-dependent boost is included in the communication apparatus, or adaptive pre-emphasis wherein the required frequency-dependent boost is calculated on-line during a periodic training sequence.
However, when noise is injected into a communications line prior to the high frequency roll-off of the communications line and the noise rolls off in parallel to the roll-off of the signals (resulting in a constant signal-to-noise ratio as a function of frequency), prior art methods of frequency-dependent analysis of total energy received are inadequate as an equalization technique.
This inadequacy is due to the fact that a positive gain is applied to higher frequency portions of the total signal. However, in order to keep the total signal energy constant as is required by telephone and other communications line applications, this positive gain in the upper frequency spectrum must be compensated for by a negative gain in the lower frequency spectrum thereby reducing the signal-to-noise ratio in the lower frequencies and resulting in a degradation in performance. This principle is illustrated in more detail by the several drawings of FIGS. 1, 2 and 3.
FIG. 1a illustrates the received transmitter signal and the received noise signal being "flat" across the entire band. The signal model for this spectrum is shown in FIG. 1b wherein the communications line includes no roll-off or other frequency-dependent characteristics and noise is added between the transmitter and the receiver. Such a system requires no frequency-dependent pre-emphasis as is shown by the ideal flat pre-emphasis of FIG. 1c.
FIG. 2a illustrates the received transmitter signal rolling off above a break frequency w.sub.o while the received noise spectrum is "flat" across the entire band. The signal included for this spectrum is shown in FIG. 2b wherein the transmitter signal passes through a channel or communications line thereby being rolled-off before having frequency-independent noise added thereto. The ideal pre-emphasis, whether manually set or periodically calculated on-line, is illustrated in FIG. 2c wherein an increasing gain is applied above the frequency w.sub.o. This frequency-dependent pre-emphasis flattens the transmitter signal as received without affecting the noise signal, thereby increasing the signal-to-noise ratio at the frequencies above w.sub.o and improving overall system performance. A slight negative gain may be applied in the lower frequencies so as to keep the total incoming energy constant. This ideal pre-emphasis is accurately calculated by prior art methods.
FIG. 3a illustrates the received transmitter signal and the received noise both rolling off in parallel above break frequency w.sub.o. The signal model for this spectrum is shown in FIG. 3c wherein the noise is added to the transmitter signal at the transmitter end and both the transmitter signal and the noise are passed through the channels or communication line thereby having substantially identical attenuation characteristics applied thereto. Therefore, the signal-to-noise ratio remains constant throughout the entire band as is illustrated by the constant vertical distance between the received transmitter signal and the noise. As the signal-to-noise ratio is constant, the ideal signal pre-emphasis is flat or shown in FIG. 3c (or 1c). However, prior art methods of pre-emphasis, either manual or automated, would look to the frequency-dependent energy spectrum of the entire received signal (received transmitter signal plus received noise) and calculate a pre-emphasis similar to that shown in FIG. 2c. This would result in a lowering of the transmitter signal power and the signal-to-noise ratio at frequencies below break frequency w.sub.o and an overall degradation in system performance.